RS53433B - PROCEDURE FOR THE PRODUCTION OF CELLULOSE-BASED FIBERS AND THE FIBERS OBTAINED THEREFORE - Google Patents

Abstract

A process for spinning a continuous fiber, comprising cellulosic nanofibrils oriented along the main axis of the fiber, from a lyotropic suspension of cellulosic nanofibrils, said nanofibril orientation being achieved by stretching the fiber extruded from a die, spinning nozzle or needle, wherein said fiber dries and dries. aggregation of oriented nanofibrils occurs in order to form a continuous fiber. The application contains another 19 patent claims.

Description

Technical field

The invention relates to the production of fibers using cellulose nanofibrils, in particular cellulose nanofibrils extracted from cellulosic material such as wood pulp.

State of the art

Cellulose is a normal chain polymer of anhydrous glucose with fi 1-4 linkages. A large number of different natural materials contain a high concentration of cellulose. Cellulose fibers in their natural form are found in such materials as cotton and hemp. Synthetic cellulose fibers are found in products such as rayon (or viscose) and in high strength fibers such as lyocell (sold under the name TENCEL™).

Natural cellulose exists in amorphous or crystalline form. During the production of synthetic cellulose fibers, cellulose is first transformed into amorphous cellulose. The strength of cellulose fibers depends on the presence and orientation of cellulose crystals, and the cellulose material can be recrystallized during coagulation processes to form a material having a given proportion of crystallized cellulose. Such fibers still contain a large amount of amorphous cellulose. For these reasons, it would be highly desirable to devise a process for obtaining cellulose-based fibers that have a high content of crystallized cellulose.

The crystallized form of cellulose can be found in wood along with other naturally occurring cellulose-based materials, which contain high-strength crystalline cellulose aggregates that contribute to the stiffness and strength of the natural material and are known as nanofibers or nanofibrils. These crystalline nanofibrils have a high strength-to-weight ratio (coefficient), which is approximately twice that of Kevlar, but today the total strength potential cannot be exploited unless these fibrils are connected into much larger crystalline units. These nanofibrils, when isolated from plant or wood cells, can have a high aspect ratio (length to width or thickness or diameter, the so-called aspect ratio) and can form lyotropic suspensions under appropriate pressures.

Song, W, Windle A. (2005): "lsotropic-nematic phase transition of dispersions of multiwall carbon nanotube" published in Macromolecules, 38, 6181-6188 describes the spinning of continuous fibers from a liquid crystalline suspension of carbon nanotubes that rapidly form a nematic phase (a long orientation arrangement per axes). The nematic structure allows good bonding between the particles within the fiber. However, natural cellulose nanofibrils once extracted from the natural material are generally from the chiral nematic phase (periodically twisted nematic structure), and when the concentration of nanofibrils is above 5 to 8% and because of this, the nanofibrils are prevented from completely orienting along the main axis of the spun fiber. Twisting in the nanofibril structure will lead to internal defects in the fiber structure.

In the article "Effect of trace electrolyte on liquid crystal type cellulose micro crystals", Longmuir, (Letter), 17 (15), 4493-4496 (2001), Araki, J. and Kuga, S. showed that bacterial cellulose can form a nematic phase in a static suspension in about 7 days. However, this approach would not be practical for the production of fibers on an industrial scale and especially in connection with the fact that bacterial cellulose is difficult and expensive to obtain.

Kimuraet al.(2005) "Magnetic alignment of the ehiral nematic phase of a cellulose microfibril suspension", Langmuir 21, 2034-2037 reported on the correction of chiral twist in a suspension of cellulose nanofibrils using a rotating magnetic field (5T for 15h) to obtain an orientation that is similar to nematic. However, this procedure is not applicable in practice for the formation of usable fibers in industry.

The work by Qizhou et al. (2006) "Transient rheological behavior of lvotropic (acetyl)

(ethyl) cellulose/m-cresol solutions" (Instantaneous rheological behavior of lyotropic acetyl/ethyl cellulose/m-cresol solutions), Cellulose 13:213-223 indicates that when the shear forces are high enough, the cellulose nanofibrils in the solution become oriented along the shear direction. The chiral nematic structure changes to a nematic-like phase, which is oriented relative to the flow. However, it was observed that the chiral nematic domain remains dispersed within the suspension Nothing is mentioned regarding the practical applications of this phenomenon, such as the formation of continuous fibers.

The work Batchelor, G. (1971) "The stress generated in non-dilute suspension of elongated particles in pure straining motion" Journal of Fluid Mechanics, 46, 813-829, investigated the application of extensional rheology to the direction of rod-shaped particle suspensions (in this case glass fibers). It has been shown that an increase in concentration, and in particular an increase in the ratio of the dimensions of rod-shaped particles, results in an increase in longitudinal viscosity. Nothing was mentioned about the possibility of correcting the chiral nematic structure present in liquid crystal suspensions.

British patent GB 1322723 filed in 1969. describes the production of fibers using fibrils. The patent is primarily focused on inorganic fibrils such as silica and asbestos, but does not mention microcrystalline cellulose as a possible, albeit hypothetical, alternative.

Microcrystalline cellulose has much larger particles than cellulose nanofibrils. It usually consists of incompletely hydrolyzed cellulose that has the form of aggregates of nanofibrils that do not immediately form lyotropic suspensions. Microcrystalline cellulose is also commonly produced using hydrochloric acid, which results in a surface charge of the nanofibrils.

GB 1322723 generally describes that fibers can be spun from a suspension containing fibrils. However, the suspensions used according to GB 1322723 have a solids content of 3% or less. This solids content is too low for any extraction. And indeed GB 1322723 refers to adding a thickening agent to the suspension. It should be noted that the application of a thickening agent would prevent the formation of a lyotropic suspension and affect the hydrogen bonds between the fibrils, which is desirable for achieving high fiber strength.

A suspension with 1 to 3% cellulose nanofibrils, especially one containing a thickening agent, would form an isotropic phase. GB 1322723 does not deal with the problems related to the use of a concentrated suspension of fibrils and in particular the use of suspensions of fibrils which are not lyotropic.

Brief content of the essence of the invention

A process is now provided which can be used to produce highly crystallized cellulosic fibers, particularly those occurring in crystalline cellulose.

The present invention relates to a process for the production of cellulose-based fibers, especially continuous (endless) fibers, which includes the stages of spinning a continuous fiber from a lyotropic suspension of cellulose nanofibrils. wherein said fiber comprises cellulose nanofibrils oriented along the main axis of the fiber, and said orientation of nanofibrils is achieved by stretching the extruded fiber from a matrix or needle and where said fiber is dried during stretching and the oriented aggregates of nanofibrils form a continuous structure.

The invention then relates to a fiber based on cellulose which contains a high degree of crystalline cellulose and can be obtained by the process according to the invention. According to a preferred embodiment of the invention, the fiber contains strictly equally oriented or continuous microstructures that give said fiber high strength.

Nanofibril extraction

Most preferably, the cellulose nanofibrils used in the invention are extracted from a cellulose-rich material.

All natural cellulose-based materials containing nanofibrils such as wood pulp or cotton can be considered as starting materials for this invention. Wood pulp is the preferred material because it has a low cost, but other cellulose-rich materials such as chitin, hemp or bacterial cellulose can be used.

Extraction of nanofibrils, as most commonly, may involve the hydrolysis of a cellulose source that is primarily ground into a fine powder or suspension.

As the most common extraction procedure involves hydrolysis with an acid such as sulfuric acid. Sulfuric acid is particularly suitable because in the hydrolysis process, charged sulfate groups are located on the surface of the nanofibrils. The surface charge of the nanofibrils creates repulsive forces between the fibers, which prevent hydrogen bonding (aggregation) in the suspension. As a result, they can slide freely between the formations of the chiral nematic liquid crystal phase in a sufficiently high concentration. The angle of the chiral nematic liquid crystal phase is determined by fibril characteristics including aspect ratio, polydispersity, and surface charge level.

Alternative nanofibril extraction procedures can be used, but the surface of the nanofibrils must be charged in order to spin continuous fibers. If the surface is not charged enough to maintain the distance between the nanofibrils during the initial phase of spinning (before drying), the aggregation of nanofibrils and eventual prevention of suspension flow during spinning may occur.

When the hydrolysis is finished, it is desirable to perform at least one phase of nanofibril fractionation, e.g. by centrifugation to remove waste fibrils and water to obtain a concentrated cellulose gel or suspension.

In order to remove as much amorphous cellulose and/or fibril waste as possible, washings can optionally be carried out as the next steps. These washing steps can be performed with a suitable organic solvent, but it is convenient to use water, especially deionized lead, followed by a separation step usually performed by centrifugation to remove fibril debris and water, since water removal is necessary to concentrate the nanofibrils. Three successive washes followed by centrifugation stages gave suitable results.

Alternatively or additionally the nanofibrils can be separated using the phase behavior of the suspension. At a critical concentration, usually around 5 to 8% of cellulose, a biphasic region is obtained, one of which is isotropic and the other anisotropic. These phases are separated according to the ratio of dimensions. At a higher ratio of fiber dimensions, an anisotropic phase is formed and it can be separated from amorphous cellulose and/or fiber waste. The relative ratio of these two phases depends on the concentration, level of surface charge and ionic content of the suspension. This procedure facilitates and/or reduces the need for centrifugation and/or washing steps. This fractionation procedure is therefore simpler and more economical (cheaper) and therefore preferred.

According to a particular embodiment of the invention, it has been found that it is convenient to adjust the zeta potential (electrokinetic potential) of the suspension using, for example, dialysis. The zeta potential can be in the range of -20 raV to -60 mV, but is preferably set in the range of -25 mV to -40 mV, preferably -28 mV to -38 mV and even more preferably -30 raV to -35 mV. To achieve this, the hydrolyzed cellulose suspension mixed with deionized water can be dialyzed using deionized water using e.g. Wisking dialysis tubing with a molecular weight cutoff in the range of preferably 12,000 to 14,000 daltons. Dialysis is used to increase and stabilize the zeta potential of the suspension from about -50 to -60 mV to preferably between -30 mV and -33 mV (see Figure 20).

This phase is particularly suitable when sulfuric acid was used to carry out the hydrolysis.

Zeta potential was determined using the Malvern Zetasizer Nano ZS system. A zeta potential lower than -30 mV results in an unstable suspension at high concentration with aggregation of nanofibrils which can lead to suspension flow interruption during spinning. A zeta potential higher than -35 mV leads to poor fiber cohesion during spinning, even at a high solids concentration above 40%.

Pressure dialysis equipment can be used to speed up the procedure.

Alternatively, dialysis of the suspension can be stopped early (eg, 3 days) and then treated with heat (to remove some of the sulfate groups) or counterions (such as calcium chloride) to reduce the zeta potential to the required level.

The nanofibril suspension may contain an organic solvent. However, it is advantageous for the said suspension to be water-based. For these reasons, the solvent or liquid phase of the suspension can be at least 90% by weight. water, primarily 95% by weight. water or even more conveniently 98% by weight. water.

According to another embodiment of the invention, it is advantageous to homogenize the cellulosic suspension before spinning in order to break up the aggregates. Ultrasound treatment (sonication) can be used, for example so that exposure is done twice for ten minutes to avoid overheating.

In order to obtain the most suitable cellulose suspension for the spinning phase, the homogenized cellulose suspension can be centrifuged again to obtain a concentrated, highly viscous suspension, which is particularly suitable for spinning. According to a preferred aspect of the invention, the cellulose suspension intended for fiber spinning is a lyotropic suspension (ie, a chiral nematic liquid crystalline phase). When the chiral twist is corrected from such a cellulose suspension, it is possible to form a highly aligned microstructure, which is desirable for obtaining a high strength fiber.

In the process according to the invention, the viscosity of the suspension required for spinning (ie, solids concentration and nanofibril aspect ratio) can vary depending on various factors. For example, it may depend on the distance between the point of extrusion and the point where the twist of the chiral structure of the fiber is corrected and then dried. A larger distance means that the wet strength and thus the viscosity of the suspension is increased. The level of concentrated solids can range from 10 to 60% by weight. However, it is convenient to use suspensions with a higher viscosity and a percentage solids content selected from the range of 20 to 50% by weight, more preferably from about 30 to 40% by weight. The viscosity of the suspension can be higher than 5000 P (poise). At these convenient concentrations, the use of thickening agents is not desirable. In any case, the minimum concentration of solid matter should be above the level at which the biphasic region occurs (in which isotropic and anisotropic phases are simultaneously present in different layers). This would typically be above 4% by weight, but more commonly above 6 to 10% by weight, depending on the ratio of nanofibril dimensions to the ionic strength of the solution. Figure 21 provides an example of the anisotropic phase volume fraction versus cellulose concentration of cotton-based cellulose nanofibrils.

Spinning of fibers from suspension

In accordance with the above, a particularly suitable implementation of the process according to the invention is carried out with a cellulose suspension in the chiral nematic phase and the spinning characteristics are defined in such a way as to straighten the twisted chiral nematic structure in the nematic phase so that it is then possible to form a continuous fiber in a scale that corresponds to industrial production in which the aggregation of nanofibrils into larger crystal structures occurs.

To spin fibers from a suspension, the cellulose suspension of nanofibrils is first fed into a needle, die, or spinning nozzle. The fiber passes through the air gap to the take-off roller which stretches it and the nanofibrils are forced to uniform orientation due to the drawing forces, and during drying of the fiber. Draw orientation is achieved by the speed of the take-off roller being greater than the speed of the fiber as it exits the die. The ratio of these two speeds is referred to as the draw down ratio (DDR-draw down ratio). The orientation (alignment) of said nanofibrils is conveniently improved by using a hyperbolic matrix designed to match the rheological characteristics of the suspension. The construction of such matrices is well documented in the publicly available literature.

When the fiber is sufficiently stretched and drawn, the bonds between the fibrils are sufficient to form larger crystalline units. By a larger crystalline unit is meant a crystallized aggregate in the range of 0.5 microns in diameter, and conveniently, up to a size corresponding to the diameter of the fiber. Preferred fiber sizes are in the range of 1 to 10 microns. Although fibers up to 500 microns or larger can be spun, the crystal unit size is not expected to exceed 5 to 10 microns. Fibers in the 1 to 10 micron range are expected to have larger crystal units and fewer crystal defects and therefore have higher strength. Larger crystal structures are formed when the draw is increased and stronger fibers are obtained when using a higher draw coefficient (DDR).

The DDR is preferably chosen to be greater than 1.2, preferably 2. Even more preferably, the DDR is greater than 3. A draw coefficient in the range of 2 to 20 is desirable for obtaining fibers with large crystalline units (above 1 micron). Extraction coefficients lower than those specified may be required to achieve higher aggregation. Draw coefficients above 5000 can be used when smaller diameter fibers are required which are obtained by reducing the initial fiber diameters from 240 microns to 1 micron, however higher draw coefficients are not necessarily required to achieve aggregation when it is required.

Drying phase

It is desirable that most of the water or solvent present in the newly formed fibers obtained by extruding through the matrix is removed during spinning. Removal of the liquid phase or drying can take a number of different forms. The most convenient approach uses heat to directly remove the liquid phase. For example, the fiber can be spun on a heated drum to achieve drying, or it can be dried using a stream of hot air, introducing (exchanging) heat, which is applied to the fiber after extrusion and preferably before it reaches the drum or take-off wheel.

An alternative approach would be to pass the wet fiber through a coagulation bath to remove most of the water, after which it can be further dried with heat.

During the drying phase, the spun fiber is stretched and the chiral nematic structure in the suspension is straightened so that the nanofibrils are oriented along the axis of the fiber in the nematic phase. When the fiber begins to dry, the nanofibrils are attached and hydrogen bonds are formed to form larger crystalline units within the fiber, maintaining the nematic phase in a solid state.

It should be noted that according to a preferred embodiment of the invention the only suspension additives besides water are counterions, such as sulfate groups, added to adjust the surface charge of the fibers.

Fiber

The fiber according to the invention primarily contains at least 90% by weight, preferably at least 95% and more, and even more preferably over 99% of crystallized cellulose. According to one variant of the invention, the fiber is formed from crystallized cellulose. Standard analytical methods including, for example, solid-state NMR (nuclear magnetic spectroscopy) or X-ray diffraction can be used to determine the relative ratio of crystalline to amorphous material.

According to a preferred embodiment of the invention, only trace amounts of amorphous cellulose (less than about 1% by weight) are present on the surface or in the core of the fiber.

According to another preferred embodiment of the invention, the fiber contains microcrystals that are highly oriented along the axial direction of the fiber. By "highly aligned/oriented" is meant that more than 95%, preferably more than 99% of the microcrystals are oriented in the axial direction. The level of alignment can be determined using electron microscopy. In addition, it is preferable that the fiber is formed from such/such microcrystals.

In addition, it is desirable that the fiber according to the present invention has a high tensile strength (relative breaking force) at least above 20 cN/tex, and more preferably in the range of 50 to 200 cN/tex.

According to the invention, the fibers can have a linear mass density (fineness), calculated according to industry standards for synthetic fibers such as Kevlar fibers or carbon fibers, in the range of 0.05 to 20 Tex (tex=g/km). Typically such fibers may have a linear mass density of about 0.5 to 1.5.

According to the following embodiment, the fiber is obtained using the process according to the invention covered by this description.

According to a particularly suitable embodiment of the invention, the process does not include the use of organic solvents, at least during the spinning phase. This feature has a particular advantage, because not using an organic solvent is not only economically profitable, but also good from an environmental point of view. Therefore, according to the features of the invention the whole process can be water-based, since the suspension used for spinning the fiber can be essentially water-based. By "substantially water-based" is meant that at least 90% by weight of the solvent used in the suspension is water. The use of water-based suspensions during the spinning process is particularly desirable due to low toxicity, low cost, easy handling and environmental benefits.

Short description of the pictures

In order to make the invention easier to understand and to practically explain it below, reference is made to the attached figures which show certain aspects of certain embodiments of the invention.

Figure 1 is a FEG-SEM (scanning emission electron microscope) image of a cellulose gel after hydrolysis and extraction by centrifugation.

Figure 2 is a FEG-SEM image of wash water after hydrolysis and extraction by centrifugation. Figure 3 is a FEG-SEM image of a cellulose gel pellet after the first wash.

Figure 4 is a FEG-SEM image of the wash water after the first wash.

Figure 5 is a FEG-SEM image of a suspension of cellulose nanofibrils after the second wash.

Figure 6 is a FEG-SEM image of the wash water after the second wash.

Figure 7 is a FEG-SEM image of the gel of cellulose nanofibrils after the third wash.

Figure 8 is a FEG-SEM image of the wash water after the third wash.

Figure 9 is a picture of the device used in example 3 for spinning the fiber.

Figure 10 is an enlarged view of Figure 9 showing the appropriate positioning of the needle and heated drum.

Figure 11 is a FEG-SEM image at 50000x magnification of a fiber spun with a small DDR. Figure 12 is a low magnification photograph of a 40 micron fiber (magnification 1000x) spun according to the invention.

Figure 13 is a FEG-SEM image of a 40 micron fiber spun according to the invention.

Figure 14 is a magnification of the image shown in Figure 13 (FEG-SEM image at 50000x magnification)

Figure 15 is a photograph at 50000x magnification showing a broken fiber according to the invention.

Figure 16 is a photograph of the underside of one of the fibers spun from DDR according to the invention. Figures 17a and 17b show the rheometer (spin line rheometer) used in example 4.

Figure 18 is a snapshot of the spun fiber obtained using the Spin line rheometer from Figure 17. Figure 19 is an enlarged snapshot from Figure 18 showing the orientation of the nanofibrils on the surface of the fiber and the location of the fiber break.

Figure 20 is a graph showing the effect of dialysis time on the zeta potential of a suspension of cellulose nanofibrils. The graph shows absolute values with a negative charge. Figure 21 is a graph showing the volume fraction of the anisotropic phase versus the cellulose concentration of the cotton-based nanofibrils after equilibration for 12 days.

Figure 22 is a comparison of polarizing microscope images of a stretched and unstretched fiber at 200x magnification. Increased birefringence can be seen in the stretched fiber indicating an oriented structure. The rough surface texture of the unstretched fiber is caused by twisted (chiral) domains, which are a permanent part of the fiber structure after cooling.

Example 1: Extraction of cellulose nanofibrils and the procedure for obtaining them

Filter paper was used as the source (raw material for) cellulose nanofibrils in this example, more precisely Whatman's cellulose filter paper no. 4. Of course the experimental conditions may vary due to different sources of cellulose nanofibrils.

The filter paper was cut into small pieces and then ground with a ball mill into a powder that could be sieved through a 20 mesh (0.841 mm).

The powder obtained by grinding on a roller mill was hydrolyzed with sulfuric acid as follows.

Cellulose powder with a concentration of 10% (w/w) was hydrolyzed using 52.5% sulfuric acid at a temperature of 46 °C for 75 min with constant stirring (using a hot plate/magnet stirrer). After the end of the hydrolysis period, the reaction was quenched by adding an amount of deionized water corresponding to ten times the hydrolysis volume.

The hydrolyzed suspension is concentrated by centrifugation with a relative centrifugal force (RCF) of 17000 for 1 h. The concentrated cellulose is then further washed 3 times and diluted after each wash using deionized water followed by centrifugation (RCF is 17000) for lh. The following example demonstrates the advantages of washing and repeated centrifugation resulting in fractionation with subsequent removal of fibril debris.

Example 2: Washing and Fractionation Study

Images of the concentrated suspension on one side as well as images of the wash water on the other side were obtained using a scanning electron microscope and emission electron microscope (FEG-SEM) to demonstrate the effect of centrifugation on the fractionation of nanofibril suspensions. After hydrolysis and extraction, three additional washes were performed. All recordings reproduced in the study are shown at a magnification of 25000x.

Hydro 1 extraction and extraction

A standard hydrolysis procedure was used for filter paper milled on a ball mill (Whatman No. 4) (sulfuric acid concentration 52.5%, 46°C and 75 minutes).

After hydrolysis of 30 grams of filter paper ground on a ball mill, the diluted suspension of nanofibrils was separated into 6 bottles of 500 ml each, which were placed in a centrifuge. The first wash is carried out for one hour at 9000 rpm (17000 G). After this time, two different phases were obtained, the acidic dissolved product of the hydrolysis (washing water) and the concentrated pellets in the gel (20% cellulose).

Figure 1 shows a FEG-SEM image of the gel structure formed after the first wash. The structure of individual cellulose nanofibrils can be seen as a clearly defined structure. However, it is very difficult to distinguish individual fibrils. This is thought to be due to the presence of amorphous cellulose and fine debris.

Figure 2 shows a FEG-SEM image of the remaining acidic solution. Individual cellulose nanofibrils could not be identified. Some structures can be seen in the image, but are indistinct, which is explained by the large amount of amorphous cellulose and fibril debris that are too small to be distinguished at this magnification.

First wash

The gel pellets were dispersed in 250 ml of deionized water for further purification in this and subsequent washes. The solution was treated in a centrifuge for one hour, the cellulose gel pellets and washing water were reevaluated. Figure 3 shows the structure of the cellulose gel after the first wash. The structure of cellulose nanofibrils is clearer than it was after the first extraction. This is explained by the extraction of most of the amorphous cellulose and fine fibril waste during the second centrifugation. Figure 4 shows a snapshot of the wash water after the first wash. It is comparable to the water in Figure 2 and is thought to still contain primarily amorphous cellulose and fine fibril debris. The amorphous character of the material is supported by the fact that it is very unstable under an electron beam. It was extremely difficult to make a recording before the destruction of the material. This problem is not observed to the same extent with crystalline nanofibrils.

Second wash

After the second wash, there does not seem to be a big difference in the structure of the nanofibrils in the cellulose gel (Figure 5) compared to the previous wash (Figure 3). However, the image of the washing water from this centrifugation (Figure 6) shows the structure better than with the previous washing water. This is explained by the elimination of most of the amorphous cellulose in the previous washing. What is left now appears to be more debris and less cellulose nanofibrils.

Third wash

After the third wash, the cellulose nanofibrils are more easily distinguishable and the gel image (Figure 7) is comparable to the wash water image shown in Figure 8. It is clear that after the second wash, most of the fine debris has been removed from the suspension and there is further loss of better quality nanofibrils. Based on this observation, a decision was made to use the suspension of cellulose nanofibrils obtained after the third washing for further processing into fibers.

The process of obtaining a suspension of cellulose nanofibrils: dialysis

At the end of the fourth centrifugation, the cellulose suspension is again diluted with deionized water and then dialyzed in deionized water using Wisking dialysis tubing with a molecular weight cutoff of 12,000 to 14,000 daltons.

Dialysis is used to reduce the zeta potential of the suspension from about -50 to -60 mV, preferably between -30 mV and -33 mV. In running deionized water, the dialysis procedure can last 2 to 3 weeks under atmospheric pressure. Figure 20 shows the results of a 4-week sample dialysis in which three lots (groups) of hydrolyzed cellulose nanofibrils were analyzed daily including analysis immediately after hydrolysis, and without dialysis (DO), for determination of zeta potential using the system: Malvern Zetasizer Nano ZS.

The data are the average of the three most recent readings with the standard deviation shown as the error bar (range) on the graph. The zeta potential data were consistent across batches and indicated that after 1 day of dialysis a relatively stable but short-lived zeta potential equilibrium between -40 and -50 mV was achieved, although with some variation shown as standard deviation. After 5 to 10 days (depending on the lot) the zeta value decreased linearly until it reached -30 mV after about 2 to 3 weeks of dialysis.

Pressure dialysis equipment can be used to speed up this process. As an alternative approach to speeding up the process, suspensions can be taken from dialysis earlier (eg 3 days) and then treated with heat (to remove some of the sulfate groups) or counterions such as calcium chloride to reduce the zeta potential to the required level.

Dialysis has particular advantages when sulfuric acid is used to perform the dialysis. Zeta potentials lower than -30 mV result in an unstable suspension with a high concentration of nanofibril aggregates, which can lead to the cessation of suspension flow during spinning. Zeta potentials above -35 mV lead to poor fiber cohesion during spinning, even at high concentrations. Poor cohesion means that the wet fiber flows as a low-viscosity fluid, which cannot be stretched and pulled before drying. A procedure that has particular advantages is the correction of chiral twist, because if the fiber is completely dried with drawing before the chiral twist is corrected, the fiber shrinks along its length, which leads to fiber breakage. When the nanofibrils are oriented along the axis of the fiber, shrinkage occurs across the width leading to a decrease in fiber diameter and an increase in coherence and strength. The nanofibrils can then also slide more easily between each other aiding the extraction process.

Dispersion and filtration

After dialysis, the cellulose preparation is sonicated using an ultrasonic device Hielscher UP200S with SI4 Type for 20 min (twice for 10 min each to avoid overheating) to disperse the aggregates. The dispersed suspension is then recentrifuged to obtain a concentrated, high viscosity suspension required for spinning.

In the first spinning example, the gel of cellulose nanofibrils was concentrated to 20% solids content using a centrifuge. In the second example, the concentration was increased to 40% in order to increase the strength of the wet gel.

Example 3: Spinning crystallized fiber on a heated drum

The first example of spinning includes the application of the device (10) shown in Figure 9 where a gel of cellulose nanofibrils is extruded from a nozzle (12) with a needle diameter of 240 microns. The injection molding process is controlled by a nozzle pump (14) attached to the housing. The fiber extruded from the injection nozzle is injected onto a ground drum (16) which can rotate at a speed of up to 1600 rpm. Drum 16 was heated to about 100 °C. The use of an automated injector pump (14) and rotating heated drum (16) allows for well-defined, controlled flow parameters and draw coefficient (DDR).

As better shown in Figure 10, the injection needle (12) is almost in contact with the heated drum (16) onto which the cellulose fibers are injected as the drum rotates, thereby creating a small air gap. The heated drum (16) provides rapid drying of the fibers which allows stretching of the fiber due to the drawing force and which leads to drawing orientation and straightening of the chiral nematic structure of the cellulose nanofibrils.

When the fiber is spun without drawing, Figure 11 shows that the orientation of the fibrils on the surface of the fiber is more or less random.

Spinning fibers with a significantly higher DDR coefficient enables better orientation of the fibrils and obtaining a thinner fiber. Table 1 below shows the details of the two flow parameters used to successfully orient the fibers, The table also gives the predicted fiber diameters which almost exactly match the obtained diameters. Manual fiber handling also shows a clear improvement in fiber strength with increasing draw coefficient. As predicted, the fiber diameter decreases with increasing draw coefficient.

Under faster draw conditions, good orientation (alignment) of fibrils was observed with a better draw coefficient. Figure 12 shows the top side of such a 40\x fiber at 1000x magnification and Figure 13 shows a FEG-SEM image of this fiber obtained with a DDR of about 4.29. The lower left edge (20) of the fiber was in contact with the heated drum (16). In the adjacent area, it is possible to see the turbulent flow of fibrils (22). The upper right side of the shot is not entirely in focus. However, it is possible to see the linear flow (nematic orientation) of the fibrils. Figure 14 shows an enlargement of the first image in the boundary regions between the turbulent flow (22) and the linear flow (24).

In order to eliminate irregularities related to drum contact drying, a different spinning equipment is used in the following example.

Figure 15 shows a broken fiber of 40 µm. It is clear from this image that the nanofibrils are oriented in a nematic structure. The figure shows that stretching the fiber before drying can successfully orient the nanofibrils. Fibers are not broken at the level of individual nanofibrils but at the level of aggregates. Aggregates are often 1 micron in size (see Figure 15 showing 1.34 and 1.27 micron aggregates (28). Aggregation occurs when nanofibrils come together due to elevated temperature.

Figure 16 shows the underside of a spun fiber with a higher draw coefficient. From the video you can see that the fiber is not completely cylindrical since it is spun on a flat drum. The drum was visibly smooth, however, there are some irregularities at the micron level that lead to cavitations (30) on the underside of the fiber when it dries. These cavitations (30) will have a large effect on the strength of the fiber and the formation of cavitations would lead to a fiber with lower strength.

An alternative approach in which the fiber exiting the die is allowed to dry without contact with some type of drum, which is used here, is described in the second spinning process in Example 4 below.

Example 4

Another example of spinning involves the use of the Spin line rheometer (32) shown in Figures 17a and 17b. This rheometer (32) has a cylinder (33) containing the cellulose suspension and is connected to the matrix (34). The extruded fiber is passed through a drying chamber (35) and dried therein using a stream of hot air before being caught by a take-up wheel (36).

The key differences between this spinning process and the process according to the previous example are as follows: - The process of extruding fibers is more precisely controlled; - When the extruded fiber is dried with warm air, as opposed to drying on a heated drum, the production of a perfect cylindrical fiber is enabled. Figure 18 shows a snapshot of the smooth surface of a 100 fiber spun from a 250 micron needle (magnification 1000x) using the rheometer of Figure 17a; -Since the fiber is air-dried, a significantly larger air gap is required to allow drying of the fiber before it is collected on the take-up wheel, which allows the fiber to be pulled out (stretched). Before spinning at high speed, the "wet" guide fiber must be pulled from the matrix and attached to form the warp. The take-off spool and die speed are increased to the point where the draw coefficient required to stretch the fiber and achieve longitudinal orientation of the fibrils can be achieved. This drawing causes the fiber to be thinned from the initial diameter of the die or needle (in this case 240 microns) to any desired fiber thickness. Ideally, the smaller the fiber, the fewer defects there are, which leads to greater strength. A fiber that has a diameter of 5 microns has a very high surface-to-volume ratio, which allows for rapid heat transfer and drying, and therefore has greater strength.

- A larger air gap means that the strength in the wet state of the nanofibril suspension must be higher than in the previous example. In order to obtain higher strength in the wet state, the content of solid matter in the suspension must be increased from 20% to 40%, which results in a much higher viscosity.

In the given example, when the nanofibril suspension is concentrated to about 40% solid matter (by centrifuging the cellulose suspension for 24 hours at 11,000 rpm), it is poured into a syringe where it is centrifuged at 5,000 rpm for 10-20 minutes to remove air pockets. The gel is then injected into the orifice of the rheometer as a single feed to prevent the formation of new air cavitations. Air pockets in the gel can lead to fiber breakage during spinning and should be avoided. The DDR coefficient used in this example was very low and amounted to about 1.5, which achieves better orientation than with a higher DDR.

Figure 19 is an enlargement of Figure 18 and shows that the nanofibrils at the break point are oriented along the fiber axis. Closer examination reveals that the nanofibrils on the fiber surface are also oriented along the fiber axis.

For illustration, Figure 22 shows polarized light microscope images of a drawn and undrawn fiber at 200x magnification. The undrawn fiber has a rough surface compared to the drawn fiber. The rough surface of the undrawn fiber is caused by periodic twisted domains resulting from chiral twisting. The collected aggregates of nanofibrils in twisted structures are of the order of a micrometer in size during drying. During drawing, the chiral twist is corrected, resulting in a smooth surface.

Other modifications will be apparent to one skilled in the art and are considered to fall within the broader scope and boundaries of the invention. In particular, the DDR coefficient can be increased to further improve the orientation of the nanofibrils and reduce the fiber diameter. This helps to reduce defects and increases the aggregation of nanofibrils into larger aggregates. And hyperbolic matrices can be constructed taking into account the rheological characteristics of the cellulose suspension for spinning. The construction of such matrices is well documented and publicly available as is the tuning mechanism of other liquid crystal solutions such as that used for lyocell.

Claims (20)

1. Process for spinning a continuous fiber, which contains cellulose nanofibrils oriented along the main axis of the fiber, from a lyotropic suspension of cellulose nanofibrils, where the stated orientation of the nanofibrils is achieved by stretching the fiber extruded from the matrix, spinning nozzle or needle, whereby the said fiber is dried during the stretching and the aggregation of oriented nanofibrils occurs to form a continuous fiber. 2. The method of claim 1, wherein said cellulose nanofibrils are extracted from a cellulose-rich material such as wood pulp or cotton. 3. The method according to claim 1 or 2, wherein the suspension is water-based. 4. A process according to any one of claims 1 to 3, wherein said process has an extraction step comprising hydrolysis of the cellulose source with an acid such as sulfuric acid. 5. The method according to any one of claims 1 to 4, wherein said extraction phase includes at least one washing phase. 6. The method according to any one of claims 1 to 5, wherein said extraction phase includes at least one separation phase to remove fibrillar waste, after the washing phase or instead of it, and which is performed by centrifugation or false separation. 7. The method according to any one of claims 1 to 6, wherein said suspension is homogenized before spinning in order to disperse aggregates. 8. The method according to any one of claims 1 to 7, wherein said fiber suspension contains cellulose nanofibrils with an average zeta potential in the range of -20 mV to -60 mV. 9. The method according to any one of claims 1 to 8, wherein said suspension contains cellulose nanofibrils with an average zeta potential in the range of -30 mV to -35 mV. 10. The process of any one of claims 1 to 9, wherein said slurry has a concentrated solids level in the range of 10 to 60% by weight. 11. The method according to any one of claims 1 to 10, wherein the extraction coefficient of the spinning phase is greater than 1.2. 12. The method of claim 11, wherein said extraction coefficient is selected to be in the range of 2 to 20. 13. A process according to any one of claims 1 to 12, wherein said process comprises spinning said suspension into fiber and wherein said extruded fiber is substantially dried during spinning. 14. The method according to any one of claims 1 to 13, wherein the orientation of said nanofibrils is improved by using a hyperbolic matrix designed to match the rheological properties of the suspension. 15. The method according to any one of claims 1 to 14, wherein said suspension is a concentrated suspension of high viscosity. 16. Cellulose-based fiber obtained by the process according to any one of claims 1 to 15. 17. Fiber based on cellulose containing at least 90% by weight. crystallized cellulose. 18. The fiber according to claim 17, wherein said fiber contains highly oriented or continuous microstructures which gives said fiber a minimum breaking strength of 20 cN/tex. 19. Fiber according to claim 17 or 18, wherein said fiber contains at least 95% of crystallized cellulose. 20. A fiber according to any one of claims 17 to 19, wherein said fiber has a linear mass density (fineness) in the range of 0.05 to 20 Tex.